Why does fiber cement siding sag?
Fiber cement siding should ventilate well, these panels "give off" moisture through the back wall.
The siding panel installed as a face plate is probably screwed tightly to the wood board and is not adequately ventilated. It has become soaked with water and has swollen a bit, causing it to warp.
Another reason is that the cut edges of the boards used for the flashing were not painted.
The main task now is to keep it from getting wet again, wait until it dries, and don't delay in staining it.